Shane Kam
Comments
This is most likely a server plugin, as you are on a server (as told by the right side)
Wrong site for this.
While yes, it does look like the advancements are gone when you head to 20w17a, simply just holding a crafting table in your inventory will unlock all of your advancements again (as the crafting table is the way to get the Minecraft advancement, which makes the actual advancement menu appear.)
If I'm not wrong, when you use "fullbright" in that client, it actually just changes the "gamma" area in the options.txt folder to a high value, which carries over to all versions that use the folder the specific options.txt is in.